Chromosomal disorders in Human

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Human cell is diploid (2n; 2 set of n; where n stands for haploid no.). n is 23 in case of human in which 22 are autosomal chromosome and 1 is sex chromosome.

  • So human cell contains 2 sets of n=23 chromosomes; 2n=46, in which 44 are autosomal chromosomes (22 sets of 2) and 2 (1 set of 2) are sex chromosomes.
  • Sex chromosomes are XX in female and XY in male.
  • Standard Nomenclature of normal individual is 46,XX for female & 46,XY for male.
  • Organisms with normal set or complete set of chromosomes are called as Euploids.
  • Organisms with extra set of chromosomes are called as (Triploid=3n having 3 sets, Tetraploid=4n having 4 sets). No genetic imbalance in organism.
  • Polyploidy is common in plants.
  • Organisms in which particular chromosome or segment of chromosome is missing/deleted or over-present/added are called as Aneuploids. Aneuploidy creates genetic imbalance in organism.
  • Cause of aneuploidy is non-disjunction of chromosomes during meiosis, or by chromosome lagging.
  • In aneuploidy , numerical change in part of genome occurs while in polyploidy numerical change in whole set of chromosomes occurs.
  • Aneuploidy can be monosomy, trisomy,nullisomy.
  • Take an example of drosophila having 8 chromosomes, i.e. 2n= 8 ,where n =4
Normal condition 2n 8
Monosomy/ monosomic condition 2n-1 7
Nullisomy 2n-2 6
Trisomy/trisomic condition 2n+1 9


  • Diploid cell with a single chromosome missing is called as Monosomic.
  • Diploid cell missing both the chromosome of a particular pair of homologous chromosome is called as Nullisomic.
  • Diploid cell with an extra chromosome is called as Trisomic.

Let’s see some examples of Aneuploidy in Humans:

Down Syndrome

  • First described by Langdon Down (1866), so called it’s name. It is most common chromosomal disorder.
  • It is TRISOMY OF 21. Presence of additional copy of chromosome no. 21.
  • Karyotype written as 47,XX, +21 in case of female or 47,XY, +21 in case of male.
  • Trisomy occurs due to non-disjunction of chromosome 21.
  • Individuals are short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease.
  • Physical, psychomotor and mental development is retarded.
  • Down syndrome is the first human syndrome to be categorized as chromosomal disorder.
  • Chances of down syndrome are more in children of older age mothers because ova remains suspended in prophase I of meiosis and non-disjunction of chromosome 21 can happen.

Edward Syndrome

  • Trisomy of chromosome no. 18
  • Karyotype is 47,XX,+18 OR 47,XY,+18
  • Mostly females get affected. Individuals have small nose and mouth, mental retardation, abnormal ears.

Patau Syndrome

  • Trisomy of chromosome no. 13
  • Karyotype is 47,XX or XY,+13
  • Individuals have cleft palate, cleft lip,polydactyly, congenital heart disease. Mortality is high in first year of life.

Klinefelter’s  Syndrome

  • It is also a case of TRISOMY, occurs due to presence of an additional copy of X chromosome in the individual.
  • Karyotype is 47,XXY. These individuals have 3 sex chromosomes, two X and one Y.
  • These individuals are phenotypically male and are sterile, but show some female secondary sexual characteristics like development of breast, i.e., Gynaecomastia. Other features are long limbs, underdeveloped body hair, small testes, behavioral disorder.
  • This condition of XXY can occur due to fertilization of XX egg with Y-bearing sperm or by fertilization of X-bearing egg with XY sperm.
  • Individuals have presence of Barr bodies in their cells.

Barr bodies are inactivated X-chromosomes that condenses into dark staining mass in the nucleus of somatic cell of mammalian females. It is a diagnostic feature of genetic femaleness.

Turner syndrome

  • Monosomic condition. Disorder due to absence of one of the X-chromosome.
  • Karyotype is 45,XO
  • These are females with underdeveloped rudimentary ovaries, abnormal jaw, webbed neck and shield-like chest. These females are sterile.
  • This condition can occur due to fertilization of X-chromosome lacking egg with normal sperm or vice-versa. Also , sometimes from loss of X-chromosome after fertilization during zygotic cleavage.
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